In order to solve exhaustion of fossil energy and earth-environmental problems by using fossil energy, research into alternative energy sources such as solar energy, wind energy, and hydro energy that are recyclable and clean has been actively conducted.
Among them, an interest in a solar cell directly converting solar lights into electric energy has significantly increased. Here, the solar cell means a cell generating current-voltage using a photovoltaic effect that the cell absorbs light energy from the solar lights to generate electrons and holes.
Currently, a n-p diode type single-crystalline silicon (Si) based solar cell having photoelectric conversion efficiency higher than 20% may be manufactured and actually used in solar power generation, and a solar cell using a compound semiconductor such as GaAs having conversion efficiency higher than that of the n-p diode type single-crystalline silicon (Si) based solar cell is present. However, since these inorganic semiconductor based solar cells require a significantly highly purified material for high efficiency, a large amount of energy is consumed in purifying a raw material, and expensive processing equipment is required during a single crystallization process or a thinning process using the raw material, such that there is a limitation in lowering a manufacturing cost of the solar cell, thereby blocking large-scale use of the solar cell.
Therefore, in order to manufacture the solar cell at low cost, a cost of a core material used in the solar cell or the manufacturing process of the solar cell should be greatly reduced, and research into a dye-sensitized solar cell (DSSC) and an organic solar cell that may be manufactured using an inexpensive material and process has been actively conducted as an alternative to the inorganic semiconductor based solar cell.
The dye-sensitized solar cell (DSSC) was initially developed by Michael Gratzel in 1991, a professor at EPFL in Switzerland and was reported in Nature.
An early dye-sensitized solar cell had a simple structure in which a dye absorbing light was loaded on porous photoanodes on a transparent electrode film through which light and electricity flow, another conductive glass substrate was positioned on the film, and a liquid electrolyte was filled therebetween.
An operation principle of the dye-sensitized solar cell is as follows. When dye molecules chemically adsorbed on surfaces of the porous photoanodes absorb solar light, the dye molecules generate electron-hole pairs, and electrons are injected into a conduction band of semiconducting oxides used as the porous photoanodes to be transported to the transparent conductive film, thereby generating current. The holes remaining in the dye molecules configure a complete solar cell circuit in a shape in which the holes are transported to photocathodes by hole conduction due to oxidation-reduction reaction of a liquid or solid electrolyte or a hole-conductive polymer, thereby performing external work.
In this dye-sensitized solar cell configuration, the transparent conductive film was mainly made of fluorine doped tin oxide (FTO) or indium doped tin oxide (ITO), and nanoparticles having a broad bandgap are used as the porous photoanodes.
As the dye, various materials capable of absorbing light particularly well and easily separating an exciton generated by the light due to a lowest unoccupied molecular orbital (LUMO) energy level that is higher than an energy level of the conduction band of the photoanode material to thereby increase the efficiency of the solar cell are chemically synthesized and used. The maximum efficiency of a liquid type dye-sensitized solar cell reported up to now has been 11 to 12% for about last 20 years. The liquid type dye-sensitized solar cell has relatively high efficiency, such that there is a possibility that the liquid type dye-sensitized solar cell will be commercialized. However, there are problems in stability according to the time by a volatile liquid electrolyte and reducing a cost due to using a high-cost ruthenium (Ru) based dye.
In order to solve these problems, research into uses of a non-volatile electrolyte using an ionic solvent rather than the volatile liquid electrolyte, a gel-type polymer electrolyte, and an inexpensive pure organic dye has been conducted, but there is a problem in that efficiency of a dye-sensitized solar cell using these materials is lower than that of the dye-sensitized solar cell using the volatile liquid electrolyte and the Ru based dye.
Meanwhile, an organic photovoltaic (OPV) that has been studied in earnest since the mid-1990 is configured of organic materials having electron donor (D, or often called a hole acceptor) characteristics and electron acceptor (A) characteristics. When the solar cell made of organic molecules absorbs the light, electrons and holes are formed, which are called exciton. The exciton moves to a D-A interface, such that an electric charge is separated, an electron moves to the electron acceptor, and the hole moves to the electron donor, thereby generating photo current.
Since a distance at which the exciton generated in the electron donor may move is about 10 nm, which is significantly short, photo active organic materials may not be thickly laminated, such that optical absorbance was low and the efficiency was low. However, recently, due to introduction of a so-called bulk heterojunction (BHJ) concept of increasing a surface area at an interface and development of an electron donor organic material having a small band gap to easily absorb solar lights in a wide range, the efficiency was significantly increased, such that an organic photovoltaic having efficiency higher than 8% has been reported (Advanced Materials, 23 (2011) 4636).
In the organic photovoltaic, a manufacturing process of a device is simple due to high formability of the organic material, diversity thereof, and a low cost thereof, such that the organic photovoltaic may be manufactured at a low cost, as compared to the existing solar cell. However, the organic photovoltaic has a problem in that a structure of the BHJ is degraded by moisture in air or oxygen, which rapidly decreases the efficiency of the solar cell, that is, a problem in the stability of the solar cell. When a technology of completely sealing the solar cell is introduced in order to solve this problem, the stability may be increased, but a cost may also be increased.
As a method of solving a problem of the dye-sensitized solar cell (DSSC) by the liquid electrolyte, an all-solid state DSSC using Spiro-OMeTAD [2,2′,7,7′-tetrkis (N,N-di-p-methoxyphenylamine)-9,9′-spirobi fluorine], which is a solid state hole conductive organic material, rather than the liquid electrolyte to thereby have efficiency of 0.74% was reported in Nature in 1998 by Michael Gratzel, a chemistry professor at EPFL in Switzerland, who is an inventor of the DSSC. Thereafter, the efficiency was increased up to about 6% by optimizing the structure, improving interfacial properties, and improving hole conductivity, and the like. In addition, a solar cell using a cheap pure organic dye instead of the ruthenium based dye and using poly(3-hexylthiophene) (P3HT), poly(3,4-ethylenedioxythiophene) (PEDOT), or the like, as a hole conductor has been manufactured, but efficiency of this solar cell is still low at 2 to 7%.
Further, research into a solar cell using quantum dot nanoparticles as a light absorber rather than the dye and using a hole conductive inorganic material or organic material rather than the liquid electrolyte has been reported. A plurality of solar cells using CdSe, PbS, or the like, as the quantum dot and using a conductive polymer such as spiro-OMeTAD or P3HT as the hole conductive organic material have been reported, but efficiency thereof were still significantly low at 5% or less. Further, a fact that a solar cell using Sb2S3 as the light absorbing inorganic material and using poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benxothiadiazole)] (PCPDTBT) as the hole conductive organic material has efficiency of about 6% has been reported [Nano Letters, 11 (2011) 4789], but it has not been reported that the efficiency is further improved.
In addition, currently, as a promising solar cell among solar cells capable of being manufactured by a solution method, a solar cell using copper zinc tin chalcogenides (CZTS/Se) or copper indium gallium chalcogenides (CIGS/Se) has been studied, but efficiency thereof was only about 11%. In addition, there were problems in that the resource such as indium is limited, a material having high toxicity such as hydrazine was used, a secondary heat treatment process at a high temperature was required, and stability and reproducibility may be deteriorated due to high volatility of chalcogen elements and it was difficult to perform long-term heat treatment at a high temperature due to the chalcogen element, such that there was a limitation in manufacturing coarse grains by the solution method.
As described above, in order to replace the semiconductor based solar cell, various solar cells such as an organic solar cell, a dye-sensitized solar cell, an inorganic quantum dot-sensitized solar cell, and an organic/inorganic hybrid type solar cell suggested by the present applicants (Korean Patent Registration No. 1172534) have been suggested, but there was a limitation in replacing the semiconductor based solar cell in view of efficiency.